ixgbe: allow to disable flow control
ixgbe: allow to disable flow control

There's a parameter "autoneg on|off" in testpmd CLI "set flow_ctrl ...". This
parameter is used to enable/disable auto negotiation for flow control. But it's
not supported yet.
The auto negotiation is enabled by default, we have no way to disable it. This
patch lets the parameter "autoneg on|off" be supproted.

Signed-off-by: Wenzhuo Lu <58cb9036bdff9a5412edc3b9f70e105ccf59954f@intel.com>
Acked-by: Changchun Ouyang <593bf1f161e79772915ea885e47cf0e95f4498c0@intel.com>

tsphillips/dpdk-fork,venkynv/dpdk-mirror,msune/dpdk,tsphillips/dpdk-fork,tsphillips/dpdk-fork,john-mcnamara-intel/dpdk,msune/dpdk,msune/dpdk,tsphillips/dpdk-fork,venkynv/dpdk-mirror,venkynv/dpdk-mirror,john-mcnamara-intel/dpdk,mixja/dpdk,msune/dpdk,mixja/dpdk,john-mcnamara-intel/dpdk,venkynv/dpdk-mirror,mixja/dpdk,john-mcnamara-intel/dpdk,mixja/dpdk